ABSTRACT: Emilianiahuxleyi, a small oceanic coccolithophore, was isolated from the NE subarctic Pacific and maintained under oceanic conditions. Coccolith-forming cultures were grown with either NO₃^- or NH₄⁺as the primary nitrogen source. Fe-stress was induced experimentally, and physiological parameters including metal quotas (Fe, Mn, Zn, Cu) were measured for both NO₃^-- and NH₄⁺-grown cells to determine whetherit was advantageous for the cells to grow on NH₄⁺ rather than NO₃^- under Fe-stressed conditions. The parameters used to observe the cell's physiological status were specific growth rate (mu), cell volume (CV),carbon (C), chlorophyll a (chl a), and nitrogen (N) per cell volume. Under Fe-replete conditions (100 nM Fe), no physiological parameters were significantly different (p &LT 0.05) between NO₃^-- andNH₄⁺-grown cells. However, under Fe-stressed conditions, CV (NH₄⁺ &GT NO₃^-), chl a CV^-1 (NH₄⁺ &LT NO₃^-), Mn CV^-1(NH₄⁺ &LT NO₃), and Mn:C (NH₄⁺ &LT NO₃^-) were all significantly different (p &LT 0.05) for NO₃^-- and NH₄⁺-grown cells. UnderFe-stressed conditions, NO₃^--grown cells of E. huxleyi maintained the same or greater levels of chl a, N, Fe, Mn, and Cu as NH₄⁺-grown cells, largely due to the drastic decrease in CV ofNO₃^--grown cells under Fe-stress. Although NO₃^--grown cells substantially decreased their CVs under Fe-stressed conditions (whereas NH₄⁺-grown cells did not), bothNO₃^-- and NH₄⁺-grown cells reduced their CVs equally under limiting-irradiance, Fe-replete conditions. A reduction in CV by the NO₃^--grown cells is particularly advantageous for cellsliving in a low Fe environment, since it reduces the cellular requirements for photosynthate, N, and Fe. The fact that these already small cells become smaller, along with their unique ability to maintain chlorophyll synthesis at Fe levels limiting tocell division, may help explain why E. huxleyi is a member of the numerically dominant size class in the NE subarctic Pacific.